Solid state relay

ABSTRACT

Solid state switching devices for selectively switching current signals between various load circuits. Normally open and normally closed transistorized switching sections incorporate noninductive driver stages adapted for high speed operation with excellent isolation between the load circuits and the control or driver circuits. The switching sections are readily combinable to form configurations which are functionally analogous to conventional electromechanical relays and no external voltages are required except normal type control voltages and the voltages which are to be switched to the various load circuits. The switching devices may be constructed to operate in either the &#39;&#39;&#39;&#39;make before break&#39;&#39;&#39;&#39; or &#39;&#39;&#39;&#39;break before make&#39;&#39;&#39;&#39; mode of operation.

United States Patent Gibbs 51 Oct. 10,1972

1 1 SOLID STATE RELAY [72] Inventor: Ralph- Stuart Gibbs, 5938 CorbinAvenue, Tarzana, Calif. 91356 [22] Filed: Feb. 1, 1971 [21] App1.No.:111,284

Related US. Application Data [63] Continuation-impart of Ser. No.855,957, Sept.

'8, 1969, abandoned.

Pickering ..307/244 Primary ExaminerJames W. Lawrence AssistantExaminer-Harold A. Dixon Attorney-Beehler, Arant & Jagger [57] ABSTRACTSolid state switching devices for selectively switching current signalsbetween various load circuits. Normally open and normally closedtransistori'zed switching sections incorporate non-inductive driverstages adapted for high speed operation with excellent isolation betweenthe load circuits and the control or driver circuits. The switchingsections are readily combinable to form configurations which arefunctionally analogous to conventional electromechanical relays and noexternal voltages are required except normal type control voltages andthe voltages which are to be switched to the various load circuits. Theswitching devices may be constructed to operate in either the makebefore break or break before make mode of operation.

25 Claims, 8 Drawing Figures PATENTEDnmmmn 3.697.772

sum 1 or 6 1 N VEN TOR.

MUQQ h v v I mm G v 7 0v vm 8, d wonwsom wmwk X Qvow J y Q SOLID STATERELAY This application is a continuation-in-part of copendingapplication for US. Letters Patent, Ser. No. 855,957, filed Sept. 8,1969' now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to solidstate switching devices and particularly to such devices which arefunctionally analogous to, and interchangeable with, conventionalelectromechanical relays.

Solid state relays offer the advantage of lower noise injection;increased relay operational life, reliability,

and switching speed; and improved performance under vibrationconditions; as well as a reduction in physical size, operating powerrequirements, and cost. Prior attempts to develop solid state relayshave been hindered by design approaches which included inductive drivingcircuits that reduce switching speed, complex circuits which lowerreliability and increased costs, or designs which provide poor isolationbetween the control circuit and the load circuit. This last mentionedisolation problem is of primary importance in computer switchingapplications, such as those for switching a source of potential throughtelephone lines to a load circuit under the control of the computersoutput circuits.

Also, if a solid state relay is to be readily interchangeable withconventional electromechanical relays, it should not require externalsupply voltages, except for a control signal and the potential to beswitched. Further, it would be desirable if the solid state relay couldbe configured to be controlled by either a single unipolar controlsignal or by a pair of complementary control signals.

Hence, a high speed solid state relay of relatively low complexity whichis readily interchangeable with electro-mechanical relays would be asignificant contribu-. tion to the art. The value of such a solid staterelay would be enhanced if it could be designed for either make beforebreak or break before make switching modes. Further, it would bedesirable that such solid state relays be adapted to economicalproduction in microminiature hybrid or integrated circuit assemblies.

SUMMARY OF THE INVENTION The devices in accordance with principles ofthe subject invention provide relatively noncomplex solid stateswitching circuitry which exhibits increased switching speed as well asexcellent isolation between driving circuits and load circuits.

In one preferred embodiment, normally open and normally closed switchingsections are combined to function as a single pole double throw relay inresponse to a control signal of a single polarity. Driver circuitsincorporated into both sections of the last mentioned polarity typeprovide the desired isolation while allowing the design choice ofparameter values so that either the make before break or break beforemake mode of. operation may be mechanized. The entire switching deviceis operable solely in response to control signals and the potential tobe switched. Hence, no additional voltage supplies are needed, and thesubject solid state devices are readily interchangeable withconventional electromechanical relays. A Darlington pair transistorcircuit is included as the load switching device for high current levelconfigurations.

In another preferred embodiment, normally open and normally closedswitching sections are combined to function as a multiple pole doublethrow relay having all the previously described features ofinterchangeability, speed,.and isolation. in such a configuration eachswitching section includes switching transistors which provide forsimultaneously switching a plurality of voltage supplies between a firstplurality of load circuits and a second plurality of load circuits undercontrol of common driver circuits incorporated in each of the sections.In the present invention, each voltage supply biases an associatedswitching transistor in each of the sections which is used for switchingthe voltage supply across its associated load circuit. Inasmuch as thepotential levels of the separate voltage supplies beingthus switchedbetween the load circuits are permitted to be different in the usualapplication of an electromechanical relay, the solid state relay of thepresent inventionprovides for isolating each of the switchingtransistors and, consequently, the load circuits associated therewith,in each of the sections, from the effects of the differences inpotentials of the respective supply voltages to assure that theswitching transistors which are supposed to be cut off are notinadvertently turned on during the operation of the relay.

Accordingly, it is a primary object of the invention to provide solidstate switching devices which are completely interchangeable withelectromechanical relays.

It is another object of the invention to provide a relatively noncomplexsolid state switching circuit that exhibits excellent isolation andincreased switching speed.

Another object is to provide basic, normally open and normally closedsolid state switching sections which may be combined to function in ananalogous manner to electromechanical relays.

A further object is to provide a solid state relay that provides anacceptable degree of isolation between the driver circuit and the loadcircuit without incorporating inductive components, and which isadaptable to microminiature hybrid and integrated circuit construction.

Still another object is to provide solid state relays comprisingnormally open and normally closed switching sections, which may befunctionally combined to operate in either the make before break orbreak before make mode of operation by the choice of parameter values inthe driver sections.

Yet a further object is to provide a solid state switching device ofrelatively simple design which is economical to manufacture and may bereliably applied to switching applications requiring high isolationbetween control and load circuits, such as in computertelephone networkinterface systems, for example.

Another object is to provide a solid state switching device which may beconfigured to be driven from high impedance, low voltage control signalsources either by a single unipolar signal or by a pair of complementarycontrol signals.

Another object of the invention is to provide a solid state relaycircuit comprised of multiple pole normally open and normally closedswitching sections in which provision is made for insuring that the loadcircuits associated with the respective poles in each of these sectionsare isolated from each other.

Another object of the invention is to provide a multiple pole doublethrow solid state relay circuit in which any one of the voltage sourcesbeing Switched by the circuit is capable of biasing the common drivercircuit associated withthe normally closed switching section of therelaycircuit.

Still another object of the invention is to provide for forward biasingthe common driving transistor associated with the normally closedswitching section of a multiple pole double throw relay by a basecurrent derived from the bias and coupling network associated with thebase terminal of any one of the switching transistors of the normallyopen switching section.

With these and other objects in view, the invention consists of theconstruction, arrangement, and combination of the various parts of thedevice, whereby the objects contemplated are attained as herein setforth, pointed out in the appended claims and illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic and blockdiagram of a solid preferred embodiment of the subject invention, whichis analogous to a single pole double throw electromechanical relay.

FIG. 2 is a schematic diagram of a solid state, normally closedswitching section which may be utilized in the device of FIG. 1 andwhich incorporates a Darlington pairtransistorized circuit ,as, the loadswitching device.

FIG. 3 is a schematic diagram of a normally open solid state switchingsection which may be utilized in the device of FIG. 1 and whichincorporates a Darlington pair transistorized circuit as the loadswitching device. a

FIG. 4 is a schematic and block diagram of a solid state switchingdevice, in accordance of one preferred embodiment of the subjectinvention, which is analogous to a double pole double throwelectromechanical relay.

FIG. 5 is a schematic and block diagram ofa solid state switching devicein accordance with another embodiment of the subject invention, whichdevice is analogous in operation to a single pole double throwelectromechanical relay and which is adapted to be controlled by a pairof complementary control signals.

FIG. 6 is a schematic and block diagram of a solid state switchingdevice of the type shown in FIG. 5 but which is functionally analogousto a double pole double throw relay.

FIG. 7 is a schematic and block diagram of a simplified version of thedouble pole double throw relay shown in FIG. 4.

'FIG. 8 is a schematic and block diagram of still another version of thedouble pole double throw relay.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the embodiment of the subjectinvention shown in FIG. 1, the switching device 10 is adapted tofunction in a manner analogous to a single pole double throwelectromechanical relay and to be interchangeable therewith in amajority of applications. The switching device 10 includes a normallyclosed switching section 12 and a normally open switching section 14 forcontrolling the application of the load current I to load re- 7 sistorsR and R respectively.

state switching device, in accordance with one Normally closed switchingsection- 12 includes a switching transistor 15 having an emitterelectrode 13 connected to a load voltage source 16, and having acollector terminal connected through a load resistor R A to a groundpotential plane 22.

Although the load, R is shown as a resistor, it should be understoodthat in accordance with the invention any type electrical circuit may bedriven as a load. Also, it is noted that although the return path forthe load current I is indicated, in the interests of simplifying the.explanation, as comprising the path through resistor R and the groundpotential plane 22, the current I need not utilize a ground bus for itsreturn. For example, in one particular application the load voltagesource 16 and the load R, are coupled by telephone lines with thecurrent 1,, flowing between positive and negative voltage sources of thetelephone network.

A base electrode 24 of the switching transistor 15 is connected througha diode 26-and a resistor 28 to the collector terminal of a drivingtransistor 30. The emitter terminal of the driving transistor 30 is connected to the ground potential plane (ground).

The voltage applied to the base electrode of the driving transistor 30isderived from a bias network which comprises a diode 32 connected betweenthe load voltage source 16 and a resistor 34. The resistor 34 isconnected to the base terminal of the driving transistor 30. Acontroltransistor 36 has collector and emitter terminals connected tothe base terminal of the driving transistor 30 and to the groundpotential plane, respectively. The base terminal of the controltransistor 36 is coupled through a resistor 40, a Zener diode 41 and adiode 43 to an actuator signal source 38. Zener diode 41 and diode 43provide improved isolation between the load voltage source 16 and theactuator signal V In the operation of the normally closed switchingsection 12 of FIG. I, the load voltage source 16 provides a positivedirect current (DC) voltage V which is applied to the emitter 13 of theswitching transistor 15, and through the diode 32 and the resistor 34 tothe base terminal of the driving transistor 30. In the absence of apositive voltage V applied from the actuator signal source 38, thebase-emitter junction of the driving transistor 30 is forward biased bycurrent that flows from the load voltage source 16 through the diode 32.In response to this forward bias current, driving transistor 30 couplesone terminal of the resistor 28 to the ground potential plane, therebyallowing bias currentv to flow in the base-emitter junction of theswitching transistor 15. In response to the bias current flowing inthejust mentioned'base-emitter junction of transistor 15, this transistoris switched to a low impedance state, hence, applying the voltage sourcepotential, .V,,, to the load resistor R During the time of applicationof a signal V from the actuator signal source 38, the switching section12 is open and the load current I is blocked by the switching transistor15 from reaching the load R,,. In the embodiment of FIG. 1, V may-be apositive signal. The actuator signal source 38 may be the output circuitof a computer that controls the line current I in a pair of telephonelines which are connected to an electromechanical print out device(analogous to R The control signal V is applied through a resistor 40 tothe base of control transistor 36. In response to this signal, thecontrol transistor 36 shorts the base of driving transistor 30 to theground reference plane, thereby causing transistor 30 to ceaseconduction, hence, interrupting the bias current through thebase-emitter junction of the switching transistor 15.

Resistor 42 is connected between the emitter terminal l3 and the baseterminal of the switching transistor to improve the turn off responseand the cutoff characteristics of the transistor. In applications whereturn off speed, leakage current and cutoff voltage are not critical, theresistor 42 may be omitted at the expense of some degradation in theperformance of these just listed features.

The resistors 28 and 34, as well as the diodes 26 and 32, provideadditional isolation between the load voltage V and the actuator signalV The diode26 also functions to protect the base-emitter junction of thetransistor 15 against excessive back bias voltage.

Normally open switch section 14 comprises a switching transistor 50having an emitter terminal coupled to the load voltage source 16 and acollector terminal coupled through the load resistor R to the groundpotential plane. The base terminal of the switching transistor 50 iscoupled through a diode 52 and a resistor 54 to the collector terminalof a driving transistor 56. The emitter terminal of the drivingtransistor 56 is connected to the ground potential plane, and its baseis biased by a bias network comprising resistors 58 and 60. These twolast mentioned res'istors are series connected between the actuatorsignal source 38 and ground.

A resistor 62a is connected between the emitter and base terminals ofthe switching transistor 50 to improve the high speed performance of thecircuit, as discussed previously relative to resistor 42.

In operation of the switching section 14, in the absence of theapplication of the control signal V,,, the driving transistor 56 isnonconducting. Hence, the switching transistor 50 is biased to thenonconductive state. Upon application of a signal V,,, the drivingtransistor 56 is biased to a low impedance state, thereby forwardbiasing the switching transistor 50 and allowing the load current I toflow through the terminal 50 to the load R In the absence of theapplication of a positive signal V to the lead 62, base currents I, and1, flow in the transistors 15 and 30, respectively, of the switchingsection 12; and little, if any, basecurrent I and 1;, flow in thetransistors 50 and 56, respectively, of the normally open section 14.

Since the turn off speed of a transistor is dependent upon the basecurrent which flows prior to the time of attempted turn off, byadjusting the parameter values of the bias elements, such as resistors28,34, and 40, the magnitude of the on current I, and I can be selectedso that the switching transistor 15 will turn off (switch to the highimpedance state) either before or after the switching transistor 50 ofthe normally closed section 14 is turned on by the application of apositive signal V By the sametoken, resistors 58, 60, and 54 may beselected such that transistor 50 will selectedly turn off either beforeor after transistor 15 is turned on upon the removal of the actuationsignal V This just discussed regulation ofthe turn on and turn offcharacteristics of the circuit 10 is important as it allows the circuitto be designed in either a make before break or'break before make"configuration. Hence, the'load current I; may be applied to the secondload before it is disconnected from the first load, or the circuit maybe so designed by selecting the values of the previously mentionedparameters that the converse switching arrangement is obtained.

The normally closed switching section 12a, shown in FIG. 2, is adaptedfor switching applications requiring high level currents and/orvoltages. In the circuit of FIG. 2, components that are similar oridentical to those of section 12 (FIG. I) are given the same referencenumeral, followed by a small letter a. The structure and function of thenormally closed switching device 12a is the same as that of the section12 of FIG. 1, except that the switching transistorlS of section 12 isreplaced by a Darlington pair .circuit 15a in the switching section 12a.

The Darlington pair circuit 15a includes transistors 64 and 66 withtheemitter andcollector terminals of the transistor 64 coupled between theload voltage source 16 (FIG. 1) and the load resistor R,,. The baseterminal of the transistor 64 is connected to the emitter terminal ofthe transistor 66 and the collector terminal of the transistor 66 isconnected to the load transistor R The base terminal of the lastmentioned transistor is connected to the driving network, which includestransistor 30a, at a junction point between the resistor 42a and thediode 26a.

The high current configuration of the normally open switch section isshown in FIG. 3 and is designated by the reference symbol 140. As wasthe case with the circuit of FIG. 2, in the circuit of FIG. 3 componentswhich are identical or similar to those of FIG. 1 are assigned the samereference numerals followed by a distinguishing small letter a. Theconstruction and operation of the high current, normally open switchingsection 14a is identical to that of section 14, except that switchingtransistor 50 of section 14 is replaced in the configuration of FIG. 3by transistors 68 and 70 connected in a Darlington pair circuitconfiguration 50a.

In the circuit 500, the emitterand collector terminals of transistor 68are connected between the load voltage source 16 (FIG. 1) and the loadresistor R The base terminal of the transistor 68 is connected to theemitter terminal of the transistor 70, and the collector terminal of thetransistor 70 is connected to the load resistor R The base terminal ofthe transistor 70 is controlled by a bias network whichincludes'resistors 62a, diode 52a, resistor 54a, and a control drivernetwork that includes the transistor 5.6a. The design and functionofthese just mentioned bias components are similar to that described fortheir corresponding counterparts of the circuit shown in FIG. 1.

The normally closed section 12a and the normally open section 14a ofFIGS. 2 and 3, respectively, may be connected in the single pole doublethrow configuration of FIG. I, as well as operating as separateswitching sections. Forexample, the sections 12 or 12a could be .used inapplications where it is desired that a voltage source be normallyconnected to a load, except during the periods of application ofacontrol signal (actuation signal). The nonnally open section 14 or 14amay be used for the switching applications in a manner similar to thatof a normally open relay.

Having described the normally closed sections, such as 12 or 12a, andthe normally open-sections, such as 14 or 14a of a single pole doublethrow solid state relay, a description will now be presented of howthese sections can be arranged to provide for a plurality of voltagesources to be respectively connected by switching transistors in each ofthe sections to a plurality of first or second load circuits. Thearrangement is such that the switching transistors in each section aredriven by a single driving transistor to thus provide synchronizedmultiple pole switching while permitting the potential levels of therespective load voltage sources to vary relative to each other withoutaffecting the reliability of the operation of the solid state relay. I

Referring now primarily to FIG. 4, the solid state switching devicethere shown is analogous to a double pole double throw electromechanicalrelay. In the configuration of FIG. 4, a first load voltage source 16bis switchable between load. resistors R and R and a second load voltagesource 16c is switchable between load resistors R and R in response toan actuation signal V applied by the actuator signal source 38.

In FIG. 4 the switching transistors associated with the sources 16b and16c, which are analogous to switching transistor 15 of FIG, 1, have beenassigned reference designations 15b and 15c, respectively. The biasingnetworks associated with these switching devices have been designated bythe reference numeral of the corresponding component in FIG. 1 with theaddition of a letter b or c. The driving circuits associated withtransistors 30c and 360 in FIG. 4 correspond to the similar circuitsassociated with transistors 30 and 36, respectively, in FIG. 1, exceptthat the collector of dn'ving transistor 30c is coupledto the inputbiasing networks of both switching transistors 15b and 150.

By the same token, the circuits associated with the switchingtransistors 50b and 500 in FIG. 4 have been designated by the samenumeral used for the corresponding component in FIG. 1, with a letteradded to distinguish the different parallel sections of the normallyopen circuits in FIG. 4. The driving transistor 56c, shown in FIG. '4,controls both switching transistors 50b and 50c.

.In operation of the double pole double throw solid state switchingdevice of FIG. 4, switching transistors 15b and 150 are normallyconducting in the absence of the actuation signal V and therefore theload voltage sources 16b and 16c are coupled to load resistors R and Rrespectively. These switching devices are driven from a common driverstage which comprises the dn'ving transistor 300. This just designatedtransistor is biased on by a current which flows through the resistor340 from the load voltage source 16b or l6cwhieh ever source is at thegreater positive potential. The diode steering network comprising diodes32b and 32c arrange for the voltage source of the higher potential to beconnected to the resistor 340. It should be noted that the purpose forthus coupling the bias circuit to both load voltage sources 16b or 160is because it is possible in a particular application or use of thesolid state relay of the present invention that only one of these loadvoltage cources will be actually connected across the terminals thereof.Hence, coupling of the bias circuit for the normally closed drivingtransistor 30c to both of the voltage load sources by way of the diodesteering network insures that transistor 30c will be biased on, such asto make the solid state relay operative, as long as any one of thevoltage sources is coupled across the terminals of the solid staterelay.

Upon application of a signal V,,, the control transistor 360 is switchedto the "on state thereby shorting the base terminal of the drivingtransistor 30c to the ground potential plane. Hence, the transistor 30cis biased out of conduction, and the flow of biasing 'current for theswitching transistors 15b and is interrupted.

The application of the actuation signal V from the signal source 38 alsoturns on driving transistor 56c, which in turn causesswitching'transistors 50b and 500 to conduct.

The make before break and break before make features, discussedpreviously, may be incorporated into the double pole double throwcircuit configuration of FIG. 4 in the same manner explained relative toFIG. li.e.', by selecting the parameters that control the base circuits:lbs I16! I21). I40, I40! Next to be discussed in connection with thedouble pole double throw solid state relay shown in FIG. 4 isthefunction of the isolating diodes 26b and 260 in the respectivecircuits coupling the base terminals of switching transistors 15b and150 to the collector terminal of driving transistor 30c, and theisolating diodes 52b and 52c in the respective circuits coupling thebase terminals of switching transistors 50b and 500 to the collectorterminal of the driving transistor 56c.

In connection herewith it should be noted that in the electromechanicalrelay art the term multiple pole indicates the number of separate,isolated circuits simultaneously controlled by contacts of the relay. Itshould be further appreciated that in a general application of anelectromechanical relay, the poles referred to could be at differentpotential levels or at least each of the line voltages selectivelysupplied to the separate circuits is normally permitted to have apotential level different from the others. Since it is not known whatthe potential levels of the voltage sources will be, all possibilitiesand variations should be accounted for in the design of a solid staterelay. It is thus seen that in order for the double pole double throwsolid state relay of the present invention to be interchangeable with aconventional electromechanical relay, the load circuits R and Rconnected to be selectively supplied by energy from load voltage source16b should thus be separate circuits isolated as completely as possiblefrom the load circuits R and R connected to be selectively supplied byenergy from load voltage source 16 c.

Inasmuch as the base circuits of the normally open switching transistors50b and 500 in FIG. 4 are coupled together to the collector of thesingle driving transistor 56c so as to be simultaneously controlledthereby, the load voltage sources 16b and 160, respectively, connectedto these switching transistors are electrically intercoupled at the baseinput circuits of the respective switching transistors. This creates aproblem inasmuch as these voltage sources when of different potentiallevel can cause current to conduct therebetween, via the base inputnetworks. Such a flow of current could modify the bias on the baseterminals of the switching transistors which are supposed to be cut off,such as to cause any one of these switching transistors to inadvertentlyconduct.

The provision of a diode 52b and 520 in the base circuit of each of theswitching transistors overcomes this problem of the load voltage sourcedifferential creating a current flow in the base circuit of thenonconducting switching transistor 50b or 50c which could inadvertentlyturn on either one of these switching transistors.

More particularly, if the diodes 52b and 52c were not present in thebase terminal circuits of the respective transistors 50b and 50c whenthese latter switching transistors are at cut-off, the switchingtransistors, and

v accordingly the lines feeding the load circuits R and R would not beisolated from each other. For example, in a particular application, ifthe voltage source 16c is at a higher potential level than the loadvoltage source 16b, in the absence of diodes 52b and 520, current wouldflow from source 16c through the resistor 620 through the resistor 54cthrough resistor 54b and resistor 62b to the load voltage source 16b.Such a current flow would cause transistor 500 to at least partiallyconduct which would result in a current flow. through to the loadR whensuch a flow is not desired.

It should be noted that in certain applications of the solid state relayin FIG. 4, the resistors 62b, 620 are not provided across theemitter-base terminals of the respective switching transistors 50b and500. In such applications, in the absence of diodes 52b and 520, thedifference in potential level of the load voltage sources 16b and 16ccan vary until the back bias of the baseemitter junction of theswitching transistor is exceeded, before creating a current flow in thebase terminal of the nonconducting switching transistor 50b or 500 whichcould inadvertently turn on either one of these switching transistors.However, a transistor typically tends to be back biased by only a fewvolts differential, and accordingly, the diodes 52b and 52c are requiredto assure that the double pole double throw solid state relay willoperate reliably over a wide range of differential voltage levels.

It should be further noted that when the solid state relay receives anactuating signal from actuator signal source 38 the switchingtransistors 50b and 50c are rendered conductive and the switchingtransistors b and 150 are now switched to cutoff. Accordingly, if diodes26b and 260 were not included in the base terminals of these latterswitching transistors, and if the load voltage source 16c were at ahigher potential level than the load voltage source 16b, this differencein potential could cause the switching transistor 15c to at leastpartially conduct, as previously discussed, thus providing a current tothe load R when it is not supposed to be energized.

In view of the above, it should now be clearly understood that thedouble pole double throw solid state relay of the present invention isintended to be used without concern as to whether the voltage sources16b and 16c happen to be of equal or different potential levels. Thus,the provision of an isolating diode in each of the base terminals of theswitching transistors insures the reliable operation of the solid staterelay by providing for isolating each of the base terminals of theswitching transistors from the higher potential of any of the loadvoltage sources applied to the common input to the driving transistor.

It should now be clear that one of the primary advantages of the subjectinvention is the increased isolation between parallel lines, e.g.,increased isolation of the line connected to the load resistor R, fromthe lines connected to the load resistors R R and R inFIG. 4. Thisimproved isolation between parallel lines is achieved by providing veryhigh impedance levels between the various lines to be switched and thecontrolling circuits; and by controlling the switching transistors, suchas transistor 15b, by means of switching their base terminals through anisolating diode-resistor network to the ground potential plane.

A solid state switching device adapted for use in conjunction with apair of complementary control signals is shown in FIG. 5. In the circuitof FIG. 5, the load voltage source 74 is switched between load resistorsR and R in response to a pair of complementary signals which aretogether designated as V,;. The signal V is supplied from an actuatorsignal source 76 on a pair of leads 77 and 78. In the embodiment, shownin FIG. 5, when the signal V is applied, the potential applied to thelead 77 is negative with respect to the ground potential plane and thesignal applied to the lead 78 is positive with respect to the groundpotential plane. In the absence of the signal V,, the transistor 78 isbiased to a low impedance state (switched 'on) and the switchingtransistor 80 is biased to a high impedance state (switched off). Uponthe application of the actuation signal V the transistor 78 is switchedoff and the transistor 80 is switched on.

The emitter and collector terminals of the transistor 78 are coupledbetween the load voltage source 74 and the resistor R The base terminalof the transistor 78 is coupled through a diode 82 and a resistor 84 tothe collector terminal of a transistor 86. The emitter terminal of thetransistor 86 is coupled to ground.

The base terminal of the transistor 86 is normally biased so that thetransistor is held on by the load voltage V applied through a diode 87and a resistor 88. Upon the application of the pair of complementarysignals V 3 negative potential is applied through a diode 91, a zenerdiode 90 andthrough the coupling resistor 92 to the base of the drivingtransistor 86, thereby switching the transistor 86 off. I

The speed up resistor 93 is connected between the emitter and the baseterminals-of the transistor 78 for the same purpose as was explainedpreviously relative to resistor 42 of FIG. 1.

The base terminal of the transistor 80 is connected through a diode 94and a resistor 96 to the collector terminal of a driving transistor 98.The emitter of the transistor 98 is connected to ground. The baseterminal of the driving transistor 98 is connected through a resistor100 to ground and also through a resistor 102, a zener diode 104 and adiode 105 to the positive lead 78 from the actuator signal source 76.

In the absence of a signal V the transistor 98 is off, henceinterrupting the flow of bias current from the base of the switchingtransistor 80 and disconnecting the load voltage source from the loadresistor R Upon the application of the signal V the driving transistor98 is switched on, thereby turning on the switching transistor 80, henceconnecting the load voltage source 74 to the load resistor R The solidstate switching device of FIG. 6 is similar in operation to the circuitjust discussed relative to FIG. 5.

However, in the configuration shown in FIG. 6, the circuit isfunctionally analogous to a double pole double throw electromechanicalrelay. In the circuit of FIG. 6, in the absence of the complementarypair of actuator signals, V,,, the switching transistors 78a and 78barebiased to a low impedance state by the transistor driving stagecomprising the transistor 86c. The bias current to the base terminal ofthe transistor 86c is supplied through the isolation resistor 88c. Theload volt age source, which supplies this just referenced bias current,is selected from the sources 74a and 74b by a steering diode networkcomprising diodes 106 and 108. This diode network connects the loadvoltage source which supplies the highest positive potential signal tothe resistor 88c. During the period of application of the signal V thenegative potential applied on the lead 77c through diode 91c, zenerdiode 90c and coupling resistor 92c biases the transistor 86c to thenonconductive state, thereby switching transistors 78a and 78b to theoff condition.

During the absence of the control signal V the transistor 98c, whichcontrols the switching transistor 80a and 80b, is biased to the offcondition. Upon the application of the signal V on the lead 78c, thedriver transistor 980 is biased into conduction, thereby providing apath for the base current of transistors 80a and 80b. Therefore, uponthe application of the signal V ,'the voltage sources 74a and 74b areconnected to load resistors R, and R respectively.

Referring next to FIG. 7, another embodiment of the double pole doublethrow solid state relay is shown. In the configuration of FIG. 7, afirst load voltage source 112a is switchable between load resistors R,and R and a second load voltage source 1121) is switchable between loadresistors R and R in response to an actuation signal source 114. In FIG.7, the normally closed switching transistors associated with the-sources112a and ll2b have been assigned reference designation 116a and 116b,respectively. The biasing network for the switching transistor 1160includes a resistor 131a connected between its emitter and base, and adiode 120a in series with a resistor 122a which couples the base ofswitching transistor 116a to the collector of driving transistor 124.Similarly, the biasing network for the switching transistor 116bincludes a resistor 13lb connected between its emitter and base, and adiode 12% in series with a resistor l22b which couples the base ofswitching transistor 116b, to the collector of driving transistor 124.

In a similar manner, the normally open switching transistors associatedwith the sources 112a and 1l2b have been assigned reference designations118a and 118b, respectively. The biasing network of the switchingtransistor 118a includes a resistor 140a connected between its emitterand base, and a diode 132a connected in series with a resistor 134awhich couples the base of switching transistor 118a to the collector ofdriving transistor 136. In a similar manner, the biasing network for theswitching transistor 1l8b includes a resistor 1401: connected betweenits emitter and base, and a diode 1321; connected in series with aresistor 134b which couples the base of switching transistor 1 18b tothe collector of the driving transistor 136.

It should be particularly noted that the circuit configuration in FIG. 7differs from that in FIG. 4 by the arrangement used to bias the drivingtransistor 124 into a normally conductive condition. Thus, the diodesteering network in FIG. 4 comprised of diodes 32b, 32c and resistor 34cconnected to the base of driving transistor c, is. eliminated in FIG. 7.This function is obtained in FIG. 7, instead, by connecting the commonjunction of the base terminal circuits of the normally open switchingtransistors 118a and l18b to the base terminal of the driving transistor124 by way of a resistor 137. It should'be thus noted that transistor124 is normally forward biased by receiving base current from the loadvoltage source 112a through resistor 140a, diode 132a, resistor 134a andresistor 137, or from load voltage source l12b through resistor 140b,diode 132b, resistor 134k and resistor 137, depending upon which of thevoltage sources 112a or ll2b is at the higher.

potential. Resistor 137 is large so that transistors 118a and 1 18bremain reverse biased.

. The circuit of FIG. 7 is actuated to be switched by a signal fromactuator signal source 114. In response to a positive signal from 114,the normally off driving transistor 136 is turned on and the controltransistor 130 is likewise turned on. The turning on of controltransistor 130 shorts the base of the driving transistor 124 to groundcausing it to be cut off and thus switching off the normally closedswitching transistor 116a and 116b and cutting off the supply of currentfrom the load' voltage source 1120 and 112b to the respective loads Rand R The simultaneous turning on of driving transistor 136 switchesswitching transistors 118a and 1181; to a conduction'condition,

thus enabling these transistors, to supply current from the load voltagesource 112a and 1 12b to the respective loads R and R Reference willnext be made to FIG. 8 which shows another circuit configuration for adouble pole double throw solid state relay, in which a first loadvoltage source 151a is switchable between load resistor R and R and asecond voltage load source 151b is switchable between load resistors Rand R This relay circuit configuration is generally similar to the oneshown in FIG. 7 but is especially adapted for switching functionsrequiring high level currents and/or voltages. This relay circuit alsoprovides for the voltage drop across the collector-emitter terminals ofthe switching transistors to be independent of the magnitude of the loadvoltage source. Thus, in FIG. 8, the normally closed switchingtransistors 116a and ll6b in FIG. 7 are replaced by transistor paircircuits 154a and 154b, and the normally open switching transistors 118aand l18b in FIG. 7 are replaced by transistor pair circuits 156a andl56b. Furthermore, each of the resistors coupled to the base terminalsof the switching transistors, such as resistor 122 a, in FIG. 7, isreplaced in FIG. 8 by a constant current diode, such as diode 169.

The transistor pair circuit 1540 includes an n-p-n transistor 160a and apn-p transistor 162a with the collector and emitter terminals of thetransistor 160a respectively coupled to the load voltage source 15 laand the load resistor R The base terminal of the transistor 160a isconnected to the collector terminal of the transistor 162a, and theemitter terminal of the transistor 162a is connected to the load voltagesource 151a. A resistor 165a is connected between the emitter and thebase terminals of the transistor 16%. The base terminal of the lastmentioned transistor is connected to the collector of a drivingtransistor 166 by way of conventional diode 167a and the constantcurrent diode 169a.

The transistor pair circuit 154b, which is similarly arranged totransistor pair circuit 154a, has the collector of its n-p-n transistor16Gb connected to load voltage source lb and the emitter of l60bconnected to load R Likewise, transistor pair circuit 156a andtransistor pair circuit l56b are similarly arranged with the formercoupling load voltage source 151a to load R and the latter coupling loadvoltage source 1511; to load R The transistor pair circuits 156a andl56b are both coupled to driving transistor 176.

The advantage of the transistor pair circuit configuration of FIG. 8over the Darlington pair circuit shown in FIG. 2 is that the former usesan n-p-n transistor, which has a higher current, higher voltage, andhigher Beta characteristic than a p-n-p transistor, for actuallyswitching the load voltage sources between the loads. Furthermore, the Vsaturation voltage is lower in an n-p-n transistor as compared to ap-n-p transistor, and this is of importance in a solid state relay inthat it results in less heat being dissipated during the operation ofthe circuit. Furthermore, the provision of the pn-p transistor foreffectively driving the n-p-n transistor has the advantage that it makesit possible for the tum on" switching voltage for the relay circuit tobe at or near ground level which is desirable in many applications. Inaddition, the replacement of the resistor in the base terminal circuitwith a constant current diode 169a, for example, provides fora constantcurrent flow in the base circuit of transistor 162a and consequently inthe base circuit of 160a, irrespective of the level of the applied loadvoltage from source 151a. Thus, the switching transistor 160a ismaintained at its optimum saturation point, and the voltage drop acrossthe collector-emitter terminals of transistor 160a remains constantirrespective of the magnitude of the applied voltage. This is ofadvantage in the solid state relay of the present invention because itmore nearly simulates the constant voltage drop across the contacts ofan electromechanical relay. The circuit configuration of FIG. 8 furthershows a driving circuit arrangement which differs from the circuit of FIG. 7 in one respect. In FIG. 8 the input 173 is provided with seriesresistors 174 and 175, the midpoint of which is connected by a capacitor177 to the reference potential plane. This arrangement provides for theactuator signal to be applied to or removed from the input with a moregradual slope, and has the effect of increasing the delay time betweenthe reversing of the bias on the base terminalsv of the drivingtransistors 166 and 176, thus assuring that the making and breakingactions of the normally open and normally closed switching sections ofthe relay occur in a controlled sequence.

Thus there has been described several preferred embodiments of solidstate switching devices which provide improved performance at a reducedcost. One of the primary advantages of these devices over prior artelectronic switches is that the subject devices do not requireadditional supply voltages for their operation. Hence, they are readilyinterchangeable with electromechanical relays. For applicationsinvolving high speed switching requirements, the subject devices arepreferable to electromechanical relays. Also, they are more economicalof operating power and maintenance costs than comparableelectromechanical relays.

Although the preferred embodiments have been described in detailherein,.it will be apparent to those skilled in the art that numerousmodifications and substitutions may be made within the scope of theinvention. In particular, although for the sake of clarity the disclosedembodiments are adapted to operate with positive source voltages, itshould be understood that the polarity of the voltages is not limiting.Voltage sources of various polarities and output voltage levels may beaccommodated by the proper selection of solid state switching devicetypes, such as field effect transistors, unipolar transistors andsilicon controlled rectifiers. Also, although in the interest ofbrevity, only single pole double throw and double pole double throwconfigurations have been shown herein, it will now be apparent that anycombination of the basic switching sections may be combined so as to befunctionally analogous to all types of electromechanical relayspresently used in the art.

What is claimed is:

l. A solid state relay for switching a source of electrical energybetween first and second load circuits in response to actuation signalsapplied thereto, said solid state relay comprising:

a first'semiconductor device having an input terminal coupled to saidenergy source and an output terminal coupled to the first load circuit,and having a control terminal;

a second semiconductor device having an input terminal coupled to saidenergy source and an output terminal coupled to the second load circuit,and having a control terminal;

a first isolating diode;

first actuator means coupled through said first isolating diode to thecontrol terminal of said first semiconductor device and controlled bysaid energy from said source of electrical energy in the absence of anactuation signal for permitting said first isolating diode to conductand causing the impedance between said input and output terminals ofsaid first semiconductor device to be at a low impedance level duringperiods of absence of said actuation signal, and controlled in responseto the actuation signal for preventing said first isolating diode fromconducting and causing said first semiconductor device to be at a highimpedance level during periods of application of the actuation signal;

a second isolating diode; and

second actuator means coupled through said second isolating diode to thecontrol terminal of said second semiconductor device and coupled to thereference potential plane of the energy source for preventing saidsecond isolating diode from conducting and causing the impedance betweensaid input and output terminals of said second semiconductor device tobe at the high impedance level during periods of absence of saidactuation signal, and controlled in response to the actuation signal forpermitting said second isolating diode to conduct and for causing theimpedance between said input and output terminals of said secondsemiconductor device to be at a low level during periods the actuationsignal is applied;

whereby said energy source is coupled to conduct through said firstsemiconductor device to said first load circuit in the absence of saidactuation signals, and said energy source is coupled to conduct throughsaid second semiconductor device to said second load circuit during theperiods of application of said actuation signals.

2. The device of claim 1 wherein said first and second semiconductordevices are transistors and said input, output and control terminals areemitter, collector and base terminals respectively.

3. The device of claim 1 wherein said first and second semiconductordevices are each Darlington pair transistor circuits.

4. The invention in accordance with claim 1 including a resistor coupledbetween the input terminal and the control terminal of each of saidfirst and second semiconductor devices.

5. The device of claim 1 wherein said first actuator means comprises:

i a driver circuit including a transistor having emitter, collector, andbase terminals, with said collector terminal coupled through said firstisolating diode to the control terminal of said first semiconductordevice, and the emitter terminal coupled to the reference potentialplane of the energy source;

first bias means coupled to said base terminal for electrically couplingsaid base terminal to the reference potential plane in response to theactuation signals; and

second bias means coupled to said base terminal for electricallycoupling said base terminal to said energy source in the absence of theactuation signal. I

6. The invention in accordance with claim 5 wherein said second biasmeans includes said second isolating diode.

7. The device of claim 5 wherein said second actuator means comprises atransistor having emitter, collector and base terminals, with the lastmentioned collector terminal coupled through said second isolating diodeto the control terminal of said second semiconductor device, the lastmentioned emitter terminal coupled to the reference potential plane ofthe energy source; and further includes means for applying the actuationsignals to the last mentioned base terminal.

8. A solid state switching device for switching a source of electricalenergy between first and second load circuits in response to actuationsignals applied thereto, said switching device comprising:

a first semiconductor device having an input temiinal and an outputterminal series coupled between the energy source and the first loadcircuit, and having a control terminal;

a second semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the'second loadcircuit, and having a control terminal;

a first isolating diode;

first actuator means coupled through said first isolating diode to thecontrol terminal of said first semiconductor device and controlled bysaid energy from said source of electricalenergy in the absence of anactuation signal for causing the im- 1 pedance between said input andoutput terminals of said first semiconductor device to be a first levelduring periods of absence of said actuation signal, and controlled inresponse to the actuation signal for causing said first semiconductordevice to be at a second impedance level during periods of applicationof the actuation signal;

a second isolating diode; and I second actuator means coupled throughsaid second isolating diode to the control terminal of said secondsemiconductor device and responding to the actuation signals for causingthe impedance between said input and output terminals of said secondsemiconductor device to be at a first level during periods the actuationsignal is applied and at the second level during periods of absence ofsaid actuation signal;

whereby said energy source is coupled through said first semiconductordevice to said first load circuit in the absence of said actuationsignals, and said energy source is coupled through said secondsemiconductor device to said second load circuit during the periods ofapplication of said actuation signals; and

wherein said first and second actuator means include means for switchingthe impedance level of the semiconductor device which is at the secondimpedance level to the first impedance level before switching the otherof said semiconductor device to the second impedance level from thefirst impedance level during the transition periods upon application orremoval of the actuation signals. 9. A solid state switching device forswitching a source of electrical energy between first and second loadcircuits in response to actuation signals applied thereto, saidswitching device comprising:

a first semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the first loadcircuit, and having a control terminal;

a second semiconductor device having an input terminal and anoutputterminal series coupled between the energy source and the secondloadcircuit, and having a control terminal;

a first isolating diode;

first actuator means coupled through said first isolating diode to thecontrol terminal of said first semiconductor device and controlled bysaid energy from said source of electrical energy in the absence of anactuation signal for causing the impedance between said input and outputterminals of said first semiconductor device to be a first level duringperiods of absence of said actuation signal, and controlled in responseto the actuation signal for causing said first semiconductor device tobe at a second impedance level during periods of application of theactuation signal;

a second isolating diode; and

second actuator means coupled through said second isolating diode to thecontrol terminal of said second semiconductor device and responding tothe actuation signals for causing the impedance semiconductor device tosaid second load circuit 1 during the periods of application of saidactuation signals; and

wherein said first and second actuator means include.

means for switching the impedance level of the semiconductor devicewhich is at the first impedance level to the second impedance levelbefore switching the other of said semiconductor device to the firstimpedance level from the second impedance level during the transitionperiods upon terminal coupled through said first isolating diode to thecontrol terminal of said first semiconductor device, and the emitterterminal coupled to the reference potential plane of the energy source;

first bias means coupled to said base terminal for electrically couplingsaid base terminal to the reference potential plane in response to theactuation signals; and

second bias means coupled to said base terminal for electricallycoupling said base terminal to said energy source in the absence of theactuation signal, said second bias means comprising a seriesdiode-resistor combination.

1 l. A solid state relay circuit for switching a first and secondvoltage source between a first pair and a second pair of load circuits,said first and second voltage sources having different potential levels,said relay circurt comprising:

a .pair .of first switching transistors, each said first application orremoval of the actuation signals. switching transistor having a controlterminal, and

10. A solid state switching device for switching a having an input andan output terminal-series cousource of electrical energy between firstand second pledbetween one of said voltage sources and its load circuitsin response to actuation signals applied associated first load circuits,and having a resistor thereto, said switching device comprising: coupledbetween the input and control terminals;

a first semiconductor device having an input terminal a pair of secondswitching transistors, each said and an output terminal series coupledbetween the second switching transistor having a control terenergysource and the first load circuit, and having minal, and having aninputand an output terminal a control terminal; series coupled betweenone of said voltages a second semiconductor device having an inputtersources and its associated second load circuits, and minal and anoutput terminal series coupled having a resistor coupled between theinput and between the energy source and the second load controlterminals; circuit, and having a control terminal; a first and seconddriving transistor, each having an a first isolating diode; inputterminal, an output terminal and a base terfirst actuator meanscoupled'through said first isolatminal, with the output terminal coupledto the ing diode to the control terminal of said first referencepotential plane of the energy sources; semiconductor device andcontrolled by said enera first coupling means including an isolatingdiode gy from said source of electrical energy in the for coupling thecontrol terminal of each said first absence of an actuation signal forcausing the im- .40 switching transistor to the input terminal of thepedance between said input and output terminals first drivingtransistor; of said first semiconductor device to be a first level asecond coupling means including an isolating diode during periods ofabsence of said actuation signal, for coupling the control terminal ofeach said and controlled in response to the actuation signal secondswitching transistor to the input terminal of for causing said firstsemiconductor device to be at the SeCOnd driving transistor;

a second impedance level during periods of applibias means coupled tothe base terminals of each of cation of the actuation signal; the firstand second driving transistors such that a second isolating diode; andthe first driving transistor is normally conducting second actuatormeans coupled through said second and the second driving transistor isnormally nonisolating diode to the control terminal of said conducting;second semiconductor device and responding to whereby the pair of firstswitching transistors couthe actuation signals for causing the impedancepled to the first driving transistor are normally between said input andoutput terminals of said conducting from the respective first and secondsecond semiconductor device to be at a first level voltage sources totheassociated first load circuits during periods the actuation signal isapplied and and the pair of second switching transistors coupled to thesecond driving transistor are normally not conducting from therespective first and second voltagesources to the associated second loadcircuits; and

input means coupled .to the base terminals of said first and seconddriving transistors capable of reversing the conduction of said firstand second driving transistors, whereby the pair of second switchingtransistors are conducting from the respective first and second voltagesources to the associated second load circuits, and the pair of firstswitching transistors are not conducting from the respective first andsecond voltage sources to the associated first load circuits; and

whereby said isolating diodes for coupling the pair of switchingtransistors to a non-conductive driving transistor are effective toprevent current flow in the control terminals of said switchingtransistors resulting from the different potential levels of saidvoltage sources and-thus prevent said switching transistors frominadvertently conducting.

12. A solid state relay circuit for switching a plurality of electricalenergy sources between a plurality of first and a plurality of secondload circuits wherein a different set of first and second load circuitsis associated with each of the energy sources, said solid state relaycircuit comprising:

a plurality of first switching transistors, each said first switchingtransistors having an input and an output terminal series coupledbetween a different one of said energy sources and its associated firstload circuit, and each of said first switching transistors having acontrol terminal;

a plurality of second switching transistors, each said second switchingtransistors having an input and an output terminal series coupledbetween a different one of said energy sources and its associated secondload circuit, and each of said second switching transistors having acontrol terminal;

a first driving transistor having an input and output terminal, and abase terminal, with the output terminal coupled to the referencepotential plane of the energy sources;

coupling means including an isolating diode for coupling the controlterminal of each of said first switching transistors to the inputterminal of said first driving transistor;

a second driving transistor having an input and output terminal, and abase terminal, with the output terminal coupled to the referencepotential plane of the energy source;

coupling means including an isolating diode for v coupling the controlterminal of each of said second switching transistors to the inputterminal of said second driving transistor;

bias means including circuit means coupling at least one of said energysources to the base terminal of said first driving transistor fornormally biasing said first driving transistor in a conductive state andsaid second driving transistor in a non-conductive state; and

input means including circuit means coupled to the base terminals ofsaid first and second driving transistors capable of reversing the stateof conduction of said first and second driving transistors;

whereby the isolating diodes coupled to the control terminals of saidfirst switching transistors are effective to enable each of the energysources coupled to its associated first load circuit through one of saidfirst switching transistors to operate as a separate isolated circuit;and

whereby the isolating diodes coupled to the control terminals of saidsecond switching transistors are effective to enable each of the energysources cou- 6 pled to its associated second load circuit through one ofsaid second switching transistors to operate as a separate isolatedcircuit.

13. The invention in accordance with claim 12 wherein said bias meansfor coupling at least one of the energy sources to the base terminal ofsaid first driving circuit includes the coupling means for coupling theplurality of second switching transistors to the collector terminal ofsaid second driving circuit.

14. A solid state switching device for switching a source of electricalenergy between first and second load circuits in response to actuationsignals applied thereto, said switching device comprising:

a first semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the first loadcircuit, and having a control terminal;

a second semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the second loadcircuit, and having a control terminal;

a first isolating diode;

first actuator means coupled through said first isolating diode to thecontrol terminal of said first semiconductor device and controlled bysaid energy from said source of electrical energy in the absence of anactuation signal for causing the impedance between said input and outputterminals of said first semiconductor device to be a first level duringperiods of absence of said actuation signal, and controlled in responseto the actuation signal for causing said first semiconductor device tobe at a second impedance level during periods of application of theactuation signal;

a second isolating diode; and

second actuator means coupled through said second isolating diode to thecontrol terminal of said second semiconductor device and responding tothe actuation signals for causing the impedance between said input andoutput terminals of said second semiconductor device to be at a firstlevel during periods the actuation signal is applied and at the secondlevel during periods of absence of said actuation signal;

whereby said energy source is coupled through said first semiconductordevice to said first load circuit in the absence of said actuationsignals, and said energy source is coupled through said secondsemiconductor device to said second load circuit during the periods ofapplication of said actuation signals; and

wherein each said first and second semiconductor devices include ann-p-n-transistor, and a p-n-p transistor, each said transistor having.collector, emitter and base terminals with the collector and emitterterminals of said n-p-n transistor corresponding to the input and outputterminal of said semiconductor device, with the collector and emitterterminals of said p-n-p transistor respectively coupled between the baseterminal of said np-n transistor and said energy source,and with thebase terminal of said p-n-p transistor corresponding to the controlterminal of said semiconductor device. I

15. A solid state switching device for switching a source of electricalenergy between first and second load circuits in response to actuationsignals applied thereto, said switching device comprising:

a first semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the first loadcircuit, and having a control terminal;

a second semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the second loadcircuit, and having a control terminal;

a first isolating diode;

first actuator means coupled through said first isolating diode to thecontrol terminal of said first semiconductor device and controlled bysaid energy from said source of electrical energy in the absence of anactuation signal for causing the impedance between said input and outputterminals of said first semiconductor device to be a first level duringperiods of absence to said actuation signal, and controlled in responseto the actuation signal for causing said first semiconductor device tobe at a second impedance level during periods of application of theactuation signal;

a second isolating diode; and

second actuator means coupled through said secon isolating diode to thecontrol terminal of said second semiconductor device and responding tothe actuation signals for causing the impedance between said input andoutput terminals of said second semiconductor device to be at a firstlevel during periods the actuation signal is applied and at the secondlevel during periods of absence of said actuation signal;

whereby said energy source is coupled through said first semiconductordevice to said first load circuit in the absence of said actuationsignals, and said energy source is coupled through said secondsemiconductor device to said second load circuit during the periods ofapplication of said actuation signals; and

wherein the coupling of each said first and second actuator means to therespective control terminals of said first and second semiconductordevices includes a constant current diode.

16. A solid state switching device for switching a plurality ofelectrical energy sources between a plurality of first and second loadcircuits wherein a different set of first and second load circuits isexclusively associated with each one of the energy sources, saidswitching device comprising:

a plurality of first semiconductor devices, each said firstsemiconductor device having an input and an output terminal seriescoupled between a different one of said energy sources and itsassociated first load circuit, and each of said first semiconductordevices having a control terminal;

a plurality of second semiconductor devices, each of said secondsemiconductor devices having an input terminal and an output terminalseries coupled between a different one of said energy sources and itsassociated second load circuit, and each of said second semiconductordevices having a control terminal;

first actuator means coupled to the control terminal of each of saidfirst semiconductor devices and controlled by the electrical energy fromthe source of electrical energy of the largest potential magnitude inthe absence ofian actuation signal for causing the impedance between theinput and output terminals of each said first semiconductor device to beat a first impedance level during the absence of the actuation signal,and controlled in response to the actuation signal for causing theimpedance level of each said first semiconductor device to be at asecond level during the periods of application of the actuation signal;

said coupling of the first actuator means to the control terminal ofeach of said first semiconductor devices including isolating means forisolating each of said first semiconductor devices suchthat the currentpath from each energy source through its associated first semiconductordevice to its associated first load circuit operates as a separate,isolated circuit;

second actuator means coupled to the control terminal of each saidsecond semiconductor-device and responding to said actuation signal forcausing the impedance level between the input and output terminals ofeach said second semiconductor device to be at a first levelduringperiods the actuation signal is applied and at a second levelduring periods of absence of said actuation signal;

said coupling of the second actuator means to the control terminal ofeach of said second semiconductor devices including isolating means forisolat ing each of said second semiconductor devices such that thecurrent path from each energy source through its associated secondsemiconductor device to its associated load circuit operates as aseparate isolated circuit;

whereby each of said plurality of energy sources is coupled through itsassociated first semiconductor device to its associated first loadcircuit in the absence of said actuation signal, and each of saidplurality of said energy sources is coupled through its associatedsecond semiconductor device to its associated second load circuit duringthe periods of application of said actuation signal.

17. The device of claim 16 wherein said first and second actuator meansincludes means for momentarily holding the impedance level between theinput and output terminals of the semiconductor devices of each of saidsets of first and second semiconductor devices which are at the firstimpedance level at that level until the impedance of the othersemiconductor device of the set is switched to the first level duringthe transition periods upon application or removal of the actuationsignal.

18. The device of claim 16 wherein said first and second actuation meansincludes means for switching the impedance level of one of thesemiconductor devices of each set of semiconductor devices from thefirst to the second impedance level before switching the othersemiconductor device of each set from the second to the first impedancelevel during the transition period upon application or removal of theactuation signal.

19. The device of claim 16 wherein said first and second actuation meansincludes means for switching the impedance level of one of thesemiconductor devices of each set of semiconductor devices from thesecond to the first impedance level before switching the othersemiconductor device of each set from the first to the second impedancelevel during the transition period upon application or removal of theactuation signal.

20. The invention in accordance with claim 16 including a resistorcoupled between the input terminal and the control terminal of each ofsaid first and second semiconductor devices.

21. The invention in accordance with claim 16 wherein said isolatingmeans for isolating each of said first and second semiconductor devicescomprises a diode connected into the control-terminal of each of saidfirst and second semiconductor devices for preventing current fiowtherein when said electrical energy sources are at different potentiallevels.

22. The invention in accordance with claim 21 wherein said switchingtransistors and said driving transistors areof complementary type.

23. The device of claim 16 wherein said first actuator means comprises:

a driver circuit including a transistor having emitter, collector andbase terminals, wherein said isolating means includes isolating diodes,and wherein said collector terminal is coupled to the control terminalof each of said first semiconductor devices through one of saidisolating diodes, and said emitter terminal of said transistor iscoupled to the reference potential plane of the energy sources;

first bias means coupled to the base terminal for electrically couplingsaid base terminal to the reference potential plane in response to theactuation signals; and

second bias means coupled to said base terminal for electricallycoupling said base terminal 'to the energy source of the largestpotential magnitude in the absence of said actuation signal.

24. The device of claim 23 wherein said second bias means comprises adiode-resistor network for connect-

1. A solid state relay for switching a source of electrical energybetween first and second load circuits in response to actuation signalsapplied thereto, said solid state relay comprising: a firstsemiconductor device having an input terminal coupled to said energysource and an output terminal coupled to the first load circuit, andhaving a control terminal; a second semiconductor device having an inputterminal coupled to said energy source and an output terminal coupled tothe second load circuit, and having a control terminal; a firstisolating diode; first actuator means coupled through said firstisolating diode to the control terminal of said first semiconductordevice and controlled by said energy from said source of electricalenergy in the absence of an actuation signal for permitting said firstisolating diode to conduct and causing the impedance between said inputand output terminals of said first semiconductor device to be at a lowimpedance level during periods of absence of said actuation signal, andcontrolled in response to the actuation signal for preventing said firstisolating diode from conducting and causing said first semiconductordevice to be at a high impedance level during periods of application ofthe actuation signal; a second isolating diode; and second actuatormeans coupled through said second isolating diode to the controlterminal of said second semiconductor device and coupled to thereference potential plane of the energy source for preventing saidsecond isolating diode from conducting and causing the impedance betweensaid input and output terminals of said second semiconductor device tobe at the high impedance level during periods of absence of saidactuation signal, and controlled in response to the actuation signal forpermitting said second isolating diode to conduct and for causing theimpedance between said input and output terminals of said secondsemiconductor device to be at a low level during periods the actuationsignal is applied; whereby said energy source is coupled to conductthrough said first semiconductor device to said first load circuit inthe absence of said actuation signals, and said energy source is coupledto conduct through said second semiconductor device to said second loadcircuit during the periods of application of said actuation signals. 2.The device of claim 1 wherein said first and second semiconductordevices are transistors and said input, output and control terminals areemitter, collector and base terminals respectively.
 3. The device ofclaim 1 wherein said first and second semiconductor devices are eachDarlington pair transistor circuits.
 4. The invention in accordance withclaim 1 including a resistor coupled between the input terminal and thecontrol terMinal of each of said first and second semiconductor devices.5. The device of claim 1 wherein said first actuator means comprises: adriver circuit including a transistor having emitter, collector, andbase terminals, with said collector terminal coupled through said firstisolating diode to the control terminal of said first semiconductordevice, and the emitter terminal coupled to the reference potentialplane of the energy source; first bias means coupled to said baseterminal for electrically coupling said base terminal to the referencepotential plane in response to the actuation signals; and second biasmeans coupled to said base terminal for electrically coupling said baseterminal to said energy source in the absence of the actuation signal.6. The invention in accordance with claim 5 wherein said second biasmeans includes said second isolating diode.
 7. The device of claim 5wherein said second actuator means comprises a transistor havingemitter, collector and base terminals, with the last mentioned collectorterminal coupled through said second isolating diode to the controlterminal of said second semiconductor device, the last mentioned emitterterminal coupled to the reference potential plane of the energy source;and further includes means for applying the actuation signals to thelast mentioned base terminal.
 8. A solid state switching device forswitching a source of electrical energy between first and second loadcircuits in response to actuation signals applied thereto, saidswitching device comprising: a first semiconductor device having aninput terminal and an output terminal series coupled between the energysource and the first load circuit, and having a control terminal; asecond semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the second loadcircuit, and having a control terminal; a first isolating diode; firstactuator means coupled through said first isolating diode to the controlterminal of said first semiconductor device and controlled by saidenergy from said source of electrical energy in the absence of anactuation signal for causing the impedance between said input and outputterminals of said first semiconductor device to be a first level duringperiods of absence of said actuation signal, and controlled in responseto the actuation signal for causing said first semiconductor device tobe at a second impedance level during periods of application of theactuation signal; a second isolating diode; and second actuator meanscoupled through said second isolating diode to the control terminal ofsaid second semiconductor device and responding to the actuation signalsfor causing the impedance between said input and output terminals ofsaid second semiconductor device to be at a first level during periodsthe actuation signal is applied and at the second level during periodsof absence of said actuation signal; whereby said energy source iscoupled through said first semiconductor device to said first loadcircuit in the absence of said actuation signals, and said energy sourceis coupled through said second semiconductor device to said second loadcircuit during the periods of application of said actuation signals; andwherein said first and second actuator means include means for switchingthe impedance level of the semiconductor device which is at the secondimpedance level to the first impedance level before switching the otherof said semiconductor device to the second impedance level from thefirst impedance level during the transition periods upon application orremoval of the actuation signals.
 9. A solid state switching device forswitching a source of electrical energy between first and second loadcircuits in response to actuation signals applied thereto, saidswitching device comprising: a first semiconductor device having aninput terminal and an output terminal series coupled between the eneRgysource and the first load circuit, and having a control terminal; asecond semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the second loadcircuit, and having a control terminal; a first isolating diode; firstactuator means coupled through said first isolating diode to the controlterminal of said first semiconductor device and controlled by saidenergy from said source of electrical energy in the absence of anactuation signal for causing the impedance between said input and outputterminals of said first semiconductor device to be a first level duringperiods of absence of said actuation signal, and controlled in responseto the actuation signal for causing said first semiconductor device tobe at a second impedance level during periods of application of theactuation signal; a second isolating diode; and second actuator meanscoupled through said second isolating diode to the control terminal ofsaid second semiconductor device and responding to the actuation signalsfor causing the impedance between said input and output terminals ofsaid second semiconductor device to be at a first level during periodsthe actuation signal is applied and at the second level during periodsof absence of said actuation signal; whereby said energy source iscoupled through said first semiconductor device to said first loadcircuit in the absence of said actuation signals, and said energy sourceis coupled through said second semiconductor device to said second loadcircuit during the periods of application of said actuation signals; andwherein said first and second actuator means include means for switchingthe impedance level of the semiconductor device which is at the firstimpedance level to the second impedance level before switching the otherof said semiconductor device to the first impedance level from thesecond impedance level during the transition periods upon application orremoval of the actuation signals.
 10. A solid state switching device forswitching a source of electrical energy between first and second loadcircuits in response to actuation signals applied thereto, saidswitching device comprising: a first semiconductor device having aninput terminal and an output terminal series coupled between the energysource and the first load circuit, and having a control terminal; asecond semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the second loadcircuit, and having a control terminal; a first isolating diode; firstactuator means coupled through said first isolating diode to the controlterminal of said first semiconductor device and controlled by saidenergy from said source of electrical energy in the absence of anactuation signal for causing the impedance between said input and outputterminals of said first semiconductor device to be a first level duringperiods of absence of said actuation signal, and controlled in responseto the actuation signal for causing said first semiconductor device tobe at a second impedance level during periods of application of theactuation signal; a second isolating diode; and second actuator meanscoupled through said second isolating diode to the control terminal ofsaid second semiconductor device and responding to the actuation signalsfor causing the impedance between said input and output terminals ofsaid second semiconductor device to be at a first level during periodsthe actuation signal is applied and at the second level during periodsof absence of said actuation signal; whereby said energy source iscoupled through said first semiconductor device to said first loadcircuit in the absence of said actuation signals, and said energy sourceis coupled through said second semiconductor device to said second loadcircuit during the periods of application of said actuation signals;wherein said first actuator means comprises: a driver circuit includinga transistor having emitter, collector, and base terminals, with saidcollector terminal coupled through said first isolating diode to thecontrol terminal of said first semiconductor device, and the emitterterminal coupled to the reference potential plane of the energy source;first bias means coupled to said base terminal for electrically couplingsaid base terminal to the reference potential plane in response to theactuation signals; and second bias means coupled to said base terminalfor electrically coupling said base terminal to said energy source inthe absence of the actuation signal, said second bias means comprising aseries diode-resistor combination.
 11. A solid state relay circuit forswitching a first and second voltage source between a first pair and asecond pair of load circuits, said first and second voltage sourceshaving different potential levels, said relay circuit comprising: a pairof first switching transistors, each said first switching transistorhaving a control terminal, and having an input and an output terminalseries coupled between one of said voltage sources and its associatedfirst load circuits, and having a resistor coupled between the input andcontrol terminals; a pair of second switching transistors, each saidsecond switching transistor having a control terminal, and having aninput and an output terminal series coupled between one of said voltagessources and its associated second load circuits, and having a resistorcoupled between the input and control terminals; a first and seconddriving transistor, each having an input terminal, an output terminaland a base terminal, with the output terminal coupled to the referencepotential plane of the energy sources; a first coupling means includingan isolating diode for coupling the control terminal of each said firstswitching transistor to the input terminal of the first drivingtransistor; a second coupling means including an isolating diode forcoupling the control terminal of each said second switching transistorto the input terminal of the second driving transistor; bias meanscoupled to the base terminals of each of the first and second drivingtransistors such that the first driving transistor is normallyconducting and the second driving transistor is normally non-conducting;whereby the pair of first switching transistors coupled to the firstdriving transistor are normally conducting from the respective first andsecond voltage sources to the associated first load circuits and thepair of second switching transistors coupled to the second drivingtransistor are normally not conducting from the respective first andsecond voltage sources to the associated second load circuits; and inputmeans coupled to the base terminals of said first and second drivingtransistors capable of reversing the conduction of said first and seconddriving transistors, whereby the pair of second switching transistorsare conducting from the respective first and second voltage sources tothe associated second load circuits, and the pair of first switchingtransistors are not conducting from the respective first and secondvoltage sources to the associated first load circuits; and whereby saidisolating diodes for coupling the pair of switching transistors to anon-conductive driving transistor are effective to prevent current flowin the control terminals of said switching transistors resulting fromthe different potential levels of said voltage sources and thus preventsaid switching transistors from inadvertently conducting.
 12. A solidstate relay circuit for switching a plurality of electrical energysources between a plurality of first and a plurality of second loadcircuits wherein a different set of first and second load circuits isassociated with each of the energy sources, said solid state relaycircuit comprising: a plurality of first switching transistors, eachsaid first switching transistoRs having an input and an output terminalseries coupled between a different one of said energy sources and itsassociated first load circuit, and each of said first switchingtransistors having a control terminal; a plurality of second switchingtransistors, each said second switching transistors having an input andan output terminal series coupled between a different one of said energysources and its associated second load circuit, and each of said secondswitching transistors having a control terminal; a first drivingtransistor having an input and output terminal, and a base terminal,with the output terminal coupled to the reference potential plane of theenergy sources; coupling means including an isolating diode for couplingthe control terminal of each of said first switching transistors to theinput terminal of said first driving transistor; a second drivingtransistor having an input and output terminal, and a base terminal,with the output terminal coupled to the reference potential plane of theenergy source; coupling means including an isolating diode for couplingthe control terminal of each of said second switching transistors to theinput terminal of said second driving transistor; bias means includingcircuit means coupling at least one of said energy sources to the baseterminal of said first driving transistor for normally biasing saidfirst driving transistor in a conductive state and said second drivingtransistor in a non-conductive state; and input means including circuitmeans coupled to the base terminals of said first and second drivingtransistors capable of reversing the state of conduction of said firstand second driving transistors; whereby the isolating diodes coupled tothe control terminals of said first switching transistors are effectiveto enable each of the energy sources coupled to its associated firstload circuit through one of said first switching transistors to operateas a separate isolated circuit; and whereby the isolating diodes coupledto the control terminals of said second switching transistors areeffective to enable each of the energy sources coupled to its associatedsecond load circuit through one of said second switching transistors tooperate as a separate isolated circuit.
 13. The invention in accordancewith claim 12 wherein said bias means for coupling at least one of theenergy sources to the base terminal of said first driving circuitincludes the coupling means for coupling the plurality of secondswitching transistors to the collector terminal of said second drivingcircuit.
 14. A solid state switching device for switching a source ofelectrical energy between first and second load circuits in response toactuation signals applied thereto, said switching device comprising: afirst semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the first loadcircuit, and having a control terminal; a second semiconductor devicehaving an input terminal and an output terminal series coupled betweenthe energy source and the second load circuit, and having a controlterminal; a first isolating diode; first actuator means coupled throughsaid first isolating diode to the control terminal of said firstsemiconductor device and controlled by said energy from said source ofelectrical energy in the absence of an actuation signal for causing theimpedance between said input and output terminals of said firstsemiconductor device to be a first level during periods of absence ofsaid actuation signal, and controlled in response to the actuationsignal for causing said first semiconductor device to be at a secondimpedance level during periods of application of the actuation signal; asecond isolating diode; and second actuator means coupled through saidsecond isolating diode to the control terminal of said secondsemiconductor device and responding to the actuation signals for causingthe impedance between said input and output terminals of said secondsemiconductor device to be at a first level during periods the actuationsignal is applied and at the second level during periods of absence ofsaid actuation signal; whereby said energy source is coupled throughsaid first semiconductor device to said first load circuit in theabsence of said actuation signals, and said energy source is coupledthrough said second semiconductor device to said second load circuitduring the periods of application of said actuation signals; and whereineach said first and second semiconductor devices include ann-p-n-transistor, and a p-n-p transistor, each said transistor havingcollector, emitter and base terminals with the collector and emitterterminals of said n-p-n transistor corresponding to the input and outputterminal of said semiconductor device, with the collector and emitterterminals of said p-n-p transistor respectively coupled between the baseterminal of said n-p-n transistor and said energy source, and with thebase terminal of said p-n-p transistor corresponding to the controlterminal of said semiconductor device.
 15. A solid state switchingdevice for switching a source of electrical energy between first andsecond load circuits in response to actuation signals applied thereto,said switching device comprising: a first semiconductor device having aninput terminal and an output terminal series coupled between the energysource and the first load circuit, and having a control terminal; asecond semiconductor device having an input terminal and an outputterminal series coupled between the energy source and the second loadcircuit, and having a control terminal; a first isolating diode; firstactuator means coupled through said first isolating diode to the controlterminal of said first semiconductor device and controlled by saidenergy from said source of electrical energy in the absence of anactuation signal for causing the impedance between said input and outputterminals of said first semiconductor device to be a first level duringperiods of absence to said actuation signal, and controlled in responseto the actuation signal for causing said first semiconductor device tobe at a second impedance level during periods of application of theactuation signal; a second isolating diode; and second actuator meanscoupled through said second isolating diode to the control terminal ofsaid second semiconductor device and responding to the actuation signalsfor causing the impedance between said input and output terminals ofsaid second semiconductor device to be at a first level during periodsthe actuation signal is applied and at the second level during periodsof absence of said actuation signal; whereby said energy source iscoupled through said first semiconductor device to said first loadcircuit in the absence of said actuation signals, and said energy sourceis coupled through said second semiconductor device to said second loadcircuit during the periods of application of said actuation signals; andwherein the coupling of each said first and second actuator means to therespective control terminals of said first and second semiconductordevices includes a constant current diode.
 16. A solid state switchingdevice for switching a plurality of electrical energy sources between aplurality of first and second load circuits wherein a different set offirst and second load circuits is exclusively associated with each oneof the energy sources, said switching device comprising: a plurality offirst semiconductor devices, each said first semiconductor device havingan input and an output terminal series coupled between a different oneof said energy sources and its associated first load circuit, and eachof said first semiconductor devices having a control terminal; aplurality of second semiconductor devices, each of said secondsemiconductor devices having an input terminal and an output terminalserieS coupled between a different one of said energy sources and itsassociated second load circuit, and each of said second semiconductordevices having a control terminal; first actuator means coupled to thecontrol terminal of each of said first semiconductor devices andcontrolled by the electrical energy from the source of electrical energyof the largest potential magnitude in the absence of an actuation signalfor causing the impedance between the input and output terminals of eachsaid first semiconductor device to be at a first impedance level duringthe absence of the actuation signal, and controlled in response to theactuation signal for causing the impedance level of each said firstsemiconductor device to be at a second level during the periods ofapplication of the actuation signal; said coupling of the first actuatormeans to the control terminal of each of said first semiconductordevices including isolating means for isolating each of said firstsemiconductor devices such that the current path from each energy sourcethrough its associated first semiconductor device to its associatedfirst load circuit operates as a separate, isolated circuit; secondactuator means coupled to the control terminal of each said secondsemiconductor device and responding to said actuation signal for causingthe impedance level between the input and output terminals of each saidsecond semiconductor device to be at a first level during periods theactuation signal is applied and at a second level during periods ofabsence of said actuation signal; said coupling of the second actuatormeans to the control terminal of each of said second semiconductordevices including isolating means for isolating each of said secondsemiconductor devices such that the current path from each energy sourcethrough its associated second semiconductor device to its associatedload circuit operates as a separate isolated circuit; whereby each ofsaid plurality of energy sources is coupled through its associated firstsemiconductor device to its associated first load circuit in the absenceof said actuation signal, and each of said plurality of said energysources is coupled through its associated second semiconductor device toits associated second load circuit during the periods of application ofsaid actuation signal.
 17. The device of claim 16 wherein said first andsecond actuator means includes means for momentarily holding theimpedance level between the input and output terminals of thesemiconductor devices of each of said sets of first and secondsemiconductor devices which are at the first impedance level at thatlevel until the impedance of the other semiconductor device of the setis switched to the first level during the transition periods uponapplication or removal of the actuation signal.
 18. The device of claim16 wherein said first and second actuation means includes means forswitching the impedance level of one of the semiconductor devices ofeach set of semiconductor devices from the first to the second impedancelevel before switching the other semiconductor device of each set fromthe second to the first impedance level during the transition periodupon application or removal of the actuation signal.
 19. The device ofclaim 16 wherein said first and second actuation means includes meansfor switching the impedance level of one of the semiconductor devices ofeach set of semiconductor devices from the second to the first impedancelevel before switching the other semiconductor device of each set fromthe first to the second impedance level during the transition periodupon application or removal of the actuation signal.
 20. The inventionin accordance with claim 16 including a resistor coupled between theinput terminal and the control terminal of each of said first and secondsemiconductor devices.
 21. The invention in accordance with claim 16wherein said isolating means for isolating each of said first and secondsemiconductor devicEs comprises a diode connected into the controlterminal of each of said first and second semiconductor devices forpreventing current flow therein when said electrical energy sources areat different potential levels.
 22. The invention in accordance withclaim 21 wherein said switching transistors and said driving transistorsare of complementary type.
 23. The device of claim 16 wherein said firstactuator means comprises: a driver circuit including a transistor havingemitter, collector and base terminals, wherein said isolating meansincludes isolating diodes, and wherein said collector terminal iscoupled to the control terminal of each of said first semiconductordevices through one of said isolating diodes, and said emitter terminalof said transistor is coupled to the reference potential plane of theenergy sources; first bias means coupled to the base terminal forelectrically coupling said base terminal to the reference potentialplane in response to the actuation signals; and second bias meanscoupled to said base terminal for electrically coupling said baseterminal to the energy source of the largest potential magnitude in theabsence of said actuation signal.
 24. The device of claim 23 whereinsaid second bias means comprises a diode-resistor network for connectingthe energy source of the largest voltage magnitude to the base terminalof the driver circuit.
 25. The device of claim 23 wherein said secondactuator means comprises a transistor having emitter, collector, andbase terminals with the collector terminal coupled to said controlterminal of each of said second semiconductor devices through one ofsaid isolating diodes, said emitter terminal coupled to the referencepotential plane of said energy sources, and including means for applyingthe actuation signal to said last mentioned base terminal.